The present invention relates to a steam inlet structure for a steam turbine, and more particularly to a steam inlet structure for the ultrahigh-temperature, high-pressure section of a ultrahigh-temperature steam turbine.
In the latter half of the 1950's several ultrahigh-temperature, high-pressure thermal power stations were constructed and went into operation, such as the Philo thermal power station unit No. 6 (steam conditions: 4500 Psig (316 atg), 1150.degree. F. (592.degree. C.)) and the Eddystone thermal power station unit No. 1 (steam conditions: 5000 Psig (352 atg), 1200.degree. F. (649.degree. C.)).
Thereafter, because these ultrahigh-temperature, high-pressure plants use large quantities of high-grade heat-resistant materials, and the cost of their equipment has greatly increased, plants with steam conditions of a maximum of 245 atg (3500 Psig) and 566.degree. C. (1050.degree. F.) have been employed.
Due to the recently increased cost of crude oil, however, attention has been again directed to ultrahigh-temperature, high-pressure plants which can provide a higher efficiency.
Because the steam turbine of an ultrahigh-temperature, high-pressure plant has a high steam inlet temperature, the ultrahigh-temperature steam inlet unit must be made of expensive heat-resistant austenite steel. But making this unit entirely of such a heat-resistant steel increases its cost, so that attempts have been made to utilize cooling techniques so that most of this unit can be made of a heat-resistant ferrite steel.
The turbine steam inlet structure and the cooling techniques used in the Eddystone thermal power station unit No. 1 are described in papers such as "The Eddystone Superpressure Unit (Transsaction of the ASME, August, 1957) and "Development Associated with the Superpressure Turbine for Eddystone Station Unit No. 1 (An ASME Publication, Paper Number 59-A-288, 1960). These papers disclose that the ultrahigh-temperature, high-pressure section is formed in a double casing consisting of an inner casing made of heat-resistant austenite steel and an outer casing made of heat-resistant ferrite steel, and the main steam inlet portion thereof is made of heat-resistant austenite steel and has a main steam inlet tube formed so that it is welded integrally to the forward end of the steam inlet of the outer casing. An annular slit is provided between the outer casing and the main steam inlet tube, and a heat-shielding plate of a special shape is arranged within this slit so as to reduce thermal stresses in the main steam inlet tube. Main steam flows into a nozzle chamber through the steam inlet tube, does its work, and is then discharged out of an exhaust pipe. Part of the discharged steam flows into the slit portion in the main steam inlet tube after passing between the inner and outer casings, to cool both the outer casing and the forward end of the steam inlet of the outer casing.
With this cooling structure, however, since the main steam inlet tube is formed so that it is welded integrally to the forward end of the steam inlet for the outer casing, difficulties have been encountered in the manufacture and inspection of the slit in the cooling steam passage, as well as of the heat-shielding plate in the slit. In particular, inspection during periodic checks is difficult. The cooling structure also has the defect that stresses are liable to concentrate at the corners of the ends of the slit.